The present invention relates generally to an improved front-end circuit for a radio transceiver, and more particularly to a front end circuit for a radio transceiver that has a common signal path for both the transmitted and received signals.
For clarity, the following specification and claims assume that a wireless communication mobile terminal includes wireless radio transceivers, such as personal communication assistance, pagers, analog and digital cellular telephones and the like, which are configured to operate in wireless communication systems where mobile terminals communicate via terrestrial and satellite base stations to any number of telephony systems.
Time Division Multiple Access (TDMA) is a time-based method for sharing communication resources in a mobile communications system. In a TDMA system, each communication channel is divided into periodic xe2x80x9cframesxe2x80x9d with each frame subdivided into several equal duration time xe2x80x9cslotsxe2x80x9d. Each mobile station is assigned a slot in the frame during which the mobile station transmits and receives information in short bursts. In a TDMA system, any given time slot may be used for transmissions in both directionsxe2x80x94mobile stationxe2x80x94toxe2x80x94base station and base stationxe2x80x94toxe2x80x94mobile station. Since there are several slots per frame, a plurality of mobile users can simultaneously use each communication channel. The assignments of timeslots is coordinated by a centrally-located xe2x80x9cmasterxe2x80x9d, which, in a in traditional wireless system, is a base station. One of the primary benefits of using a Time-Division Multiple Access (TDMA) is to allow multiple users to time-share a limited radio frequency spectrum.
Time-Division Duplex (TDD) is a variation of the TDMA concept. In a TDD system, specific slots in time are designated for only portables to transmit and base stations to receive (and vice versa). With the hardware operating under these constraints, radio hardware designers are often able to share radio circuitry common to both transmitter and receiver, since signals are only present in either the receive or the transmit direction at any one time. This provides benefits of reduction of circuitry, resulting in reduced cost, smaller size and reduced complexity.
Included in the circuitry shared between receiver and transmitter is the antenna and radio frequency (RF) signal filtering. These items perform similar functions in the receive and transmit modes, but at vastly different signal levels. The receiver is designed for detecting and processing extremely small signals (on the order of pico Watts (10xe2x88x9212 Watts)), and for that, semiconductor devices with great sensitivity are required. The transmitter, however, is producing power levels typically twelve or more orders of magnitude greater than thatxe2x80x94typically 1 Watt. The large signals can damage the much more sensitive receiver circuitry, if the receiver is not protected, or isolated in some way. Thus, some sort of a switching function is needed for such a system.
A variety of solutions have been used over the years in which receiver and transmitters have shared antenna and filtering subsystems. Passive RF power combiners, for example, are designed such that the signal can be routed to each path at the same time, while maintaining isolation between transmitter and receiver. However, in general, they are not a good solution for this problem, as half the power goes to each path all the time. Since each circuit in a TDMA or TDD system is only used half the time that the radio is active, this is very inefficient. RF circulators, comprised of ferrite devices which pass signals in one direction with low loss, but in the opposite with high loss are another option. Until only recently such systems were not suitable for use in mobile radio transceivers because their size, weight and cost were prohibitive. Also, RF circulators might not by themselves provide sufficient performance.
Consequently, a transmit/receive (T/R) RF switch is most often present near the antenna of the radio for TDMA and TDD systems. This T/R switch is generally a single-pole-double-throw (SPDT) switch, but depending upon the presence and extent of antenna and/or other diversity devices, more poles and xe2x80x9cthrowsxe2x80x9d may be present.
A block schematic of a mobile communications device is shown in FIG. 1. These systems generally include a transmitter 10 and receiver 12 alternately switched onto a common signal path 14 by an RF T/R switch 16. The common signal path 14 leads to an antenna 18 and preferably includes a filter 20, which is generally a bandpass filter (BPF). The transmitter 10 will usually include a power amplifier 22 while the receiver 12 will usually include a low-noise amplifier (LNA) 24.
The RF switches in the earliest days of radio were electromechanical relay switches, which were slow to settle between states and suffered from early fatigue, since they were generally limited to less than 1 million operations. As semiconductors improved in technology, bandwidth, and cost, they became a much more attractive solution. Currently, with the speed and reliability required for modern systems (PWT/DECT switches at least 200 times per second during a normal phone call), solid-state (semiconductor) switches are necessary.
Several topologies of RF switch designs using xe2x80x9cPiNxe2x80x9d diodes as the switching elements are commonly used. These diodes exhibit a distinct feature that when forward-biased, they appear as a short circuit to radio frequencies, and when not biased (or reverse-biased) appear as a very high impedance to RF. In RF switches, PiN diodes are alternately biased on and off to produce short and open circuits, respectively, in different arrangements to form SPST, SPDT and DPDT RF switches.
FIG. 2(a) shows one of the more commonly-used topologies for the RF switch 16, the series-shunt configuration. This configuration generally includes a series PiN diode 26 and a shunt PiN diode 28. Both diodes are biased on simultaneously during the transmit mode through an inductor 30 by a control signal, such as VBias. The block diagrams in FIG. 2(b) shows the effective functionality of this circuit. Note that although the shunt diode 28 is biased ON, and thus low resistance, its appearance to the antenna in the transmit mode is that of an open circuit, due to the impedance transformation occurring via the quarter-wavelength (xcex/4) transmission line 32. The transmission line length may be actual or effective based on inductance and capacitance affecting or added to the signal path.
Great strides have also been made in recent years to bring the cost of Gallium Arsenide (GaAs) components to a level more competitive with silicon and thus GaAsFET (GaAs field effect transistor) RF switch solutions have become another widely used solution, generally in the form of a Monolithic Microwave Integrated Circuit (MMIC). The MESFET""s are also biased on or off to produce either low or very high resistances to radio frequency signals. The most common circuit topology used is a xe2x80x9cbranchedxe2x80x9d T-pad configuration shown in FIG. 3(a), where in the xe2x80x9cONxe2x80x9d arm of the switch, the series FETs 40, 42 are ON (low resistance) and the shunt (to-ground) FET 44 is OFF (high-impedance). In contrast, the OFF arm of the switch has the two series FETs 46, 48 presenting a high series impedance to signal, while also shorting the signal to ground with the shunt FET 50 between the two series devices 46, 48. FIG. 3(b) shows the effective operation of the circuit in a transmit mode. The switch positions are reversed during the receiving operation.
Most of the RF switches are designed and specified to operate in a 50-Ohm environment, which has become the most common standard, due to the ease of implementing passive and active circuits at practically any radio frequency at this impedance. The loss of signal through the intended signal paths (antenna to receiver and transmitter to antenna), called insertion loss, is a parameter of importance for these devices. But without adequate reduction of signal level between receiver and transmitter (called signal isolation), the low-noise amplifier (LNA) in the receiver front-end can be permanently damaged by the radio""s own transmitted signal. For example, damage levels for such devices typically occur at a level as low as +10 dBm, which would require a 1-watt transmitter (+30 dBm) to be isolated by at least 20 dB.
With the transmitter 10 connected to one of the switch branches and the receiver connected to the other, a non-ideal switch will permit a finite amount of signal to leak between these parts of the radio. Of most concern is signal leakage during transmit mode. The transmitter""s relatively high power signal, which is typically on the order of 100 mW to in excess of 1 Watt, may cause damage to or degrade the performance of the receiver""s LNA 24 when leakage occurs.
Even if not sizable enough to cause damage, a leakage signal from the transmitter 10 could still be sufficient enough in magnitude to find its way through shared circuitry, especially via local oscillator signals shared between transmitter upconverter(s) and receiver downconverter(s). This phenomenon is essentially feedback into the transmitter xe2x80x9cloopxe2x80x9d and can result in transmitter instability, such as oscillation, and other signal distortion when the leakage signal at the same frequency is reamplified and combined at varying phase angles and/or delays (determined by the exact coupling path-length) with the desired transmitted signal.
The transmitter xe2x80x9cfeedbackxe2x80x9d problem described above can become more prevalent when the LNA 24 is powered down in the transmit mode, normally resulting in a high impedance state at its input. This allows a larger voltage swing on the LNA input pin than when terminating the signal in the xe2x80x9cnormalxe2x80x9d 50-Ohms when the LNA is powered on, thus resulting in signal isolation that is lower than expected. Typically, the isolation commonly achievable for the GaAs MMIC switches described above can have values as little as 12 dB, with more superior devices yielding 20 to 30 or more dB of isolation. However, with a high-impedance load, the high-impedance series FET on the LNA side does not drop the signal""s voltage amplitude as much as it would with a low-impedance load (i.e., 50 Ohms), thus producing less than the specified isolation.
Amplifying the situation even further is the increasingly common use of a single transceiver Application-Specific Integrated Circuit (ASIC), where the LNA 24, a transmitter amplifier preceding the power amplifier 22, and RF up- and downconverters share the same small silicon die and package. This presents great concern, as these circuits are physically located within millimeters (or less) of one another, with bondwires running potentially parallel to each other, resulting in significant amounts of inductive coupling, a situation not present in other embodiments. The leakage signal may produce a significant voltage swing that can be picked up by other active circuits within the package. Again, since the leakage signal often arrives via a path with significantly varying phase over frequency, it can combine with the desired signal in-phase and out-of-phase, resulting in constructive and destructive amplitude response, which can be seen across the transmitter""s passband.
The present invention provides an effective and economical solution for the problems described above. The invention relates to an improved front-end circuitry for a mobile terminal for which the receiver and transmitter share a common transmission path, which is switched between the receiver and transmitter during operation via an RF Transmit/Receive (T/R) switch. An additional shunt switch is placed between the T/R switch and the receiver circuitry, and, preferably, in close proximity to the receiver circuitry. The shunt switch is configured to provide a low impedance electrical path to ground for the receiver circuitry input during transmission sequences and an open circuit during receive sequences. Coupling the receiver input to ground during transmission sequences at a location proximate to the receiver input significantly minimizes the amplitude of all unwanted signals appearing at the receiver""s input during transmission sequences, especially attenuated and/or delayed replicas of the transmit signal itself. These signals can result in distortion of the transmitted signal due to feedback into the transmitter chain and/or damage to the sensitive electronics of the receiver.
Accordingly, the present invention relates to a transceiver, such as a mobile radio transceiver, and particularly to front-end circuitry coupled between an antenna and a receiver and transmitter. The front-end circuitry includes a common signal path between an RF Transmit/Receive (T/R) switch and the antenna wherein the T/R switch has a first state coupling the transmitter to the common signal path and a second state coupling the receiver to the common signal path via a receive signal path. A shunt switch is provided between the receive signal path and ground wherein the receive signal path is coupled to ground during transmission sequences to minimize the amplitude of all unwanted signals appearing at the receiver""s input during transmission sequences, especially attenuated and/or delayed replicas of the transmitted signal itself. These signals can result in distortion of the transmitted signal due to feedback into the transmitter chain and/or damage to the sensitive electronics of the receiver. The shunt switch is preferably any type of solid state switching circuitry, including various types of transistors or diodes arranged alone or in combination to function as a switch based on a control signal corresponding to transmission and receive sequences. Preferably, the shunt switch is placed at an electrical distance less than an equivalent one-eighth wavelength of the carrier frequency along the transmission line from the receiver input, which is typically a low-noise amplifier. Although this preferred placement is not required, attenuation of the stray signals increases as the distance between the shunt switch and receiver input decreases. It is also preferable to place the shunt switch at an electrical distance of approximately one-quarter wavelength of the carrier frequency from the T/R switch.
The present invention is ideal for Time-Division Multiple Access (TDMA) and Time-Division Duplex (TDD) radio communications systems. The invention consists of electronics located within the front-end circuitry of a radio transceiver, the implementation of which can be comprised of discrete circuitry and/or integrated circuit (IC) technology.
These and other aspects of the present invention will become apparent to those skilled in the art after reading the following description of the preferred embodiments when considered with the drawings.